I was having this same problem the other day, but found it only is a problem some of the time.
I made a simple form with just a few buttons, and it worked. Then I added a combo and it still worked. But then I added an edit box, which messed it up.
If the buttons have the initial focus, then it seems to work fine, but as soon as I tab down into the edit box (or click in it with the mouse), it wont accept any more button input via keyboard (alt-s, etc). Nor can I use tab to move from the edit box to another control.
I haven't worked it out beyond that.